Continuously variable transmissions (CVT) of the variable pulley or sheave type, employ pulley assemblies having at least one member that is moveable to control the diameter at which a flexible transmitter, such as a belt or chain, operates. The transmission has an input pulley and an output pulley, both of which have an adjustable member. The transmission ratio between the input and the output pulleys varies between an underdrive ratio and an overdrive ratio.
The CVT ratio is continuously variable between the extremes of the underdrive and overdrive ratios. During the underdrive ratios, the flexible transmitter is positioned at a small diameter on the input pulley and a large diameter on the output pulley. Thus, the input pulley has more than one revolution for each revolution of the output pulley. As the diameter of the input pulley increases, the diameter of the output pulley decreases until a 1:1 ratio exists across the pulleys. During the overdrive ratios, the diameter of the input pulley is maintained larger than the diameter of the output pulley. Thus, each revolution of the input pulley results in more than one revolution of the output pulley.
To accommodate the ratio variance, at least one member of each of pulley is disposed to slide axially relative to the other member of the pulley. This movement is typically controlled hydraulically.
CVTs have become increasingly popular in recent years because they may provide improved fuel economy, the ability to operate the engine at lower rpms over a wider range of the fuel economy schedule, smoother shifting (ratioing), more efficient vehicle front end packaging, as well as manual transmission interchangeability and all-wheel drive compatibility. In vehicles with higher horse power engines, CVTs are required to have higher torque capacity and excellent durability or wear resistance of components. The high torque carrying capacity is achieved by maintaining high frictional engagement between the belts and pulleys. Durability is achieved by limiting wear resistance of the pulley surfaces.
It has been found that pulleys with higher roughness show higher friction carrying capability with minimal slippage. Accordingly, the surface of the pulley is usually engineered to a high roughness average (Ra), measured in a radial direction with respect to the axis of rotation of the pulley, which is also referred to as a “centerline average roughness.” By way of example, U.S. Pat. No. 6,254,503 teaches a pulley having a centerline roughness average (Ra) of 0.1 to 0.5 microns, and a Vickers hardness of not less than 850 at a load of 200 g. However, the control of average roughness (Ra) in a radial direction of the pulley has proved to be ineffective.